Built-up roofing (BUR) systems generally comprise a substantially rigid deck covered with a membrane comprising multiple layers of bitumen impregnated felt having a separately applied coating of bitumen on top of each layer of felt with a protective layer of small stones or other inert mineral aggregate materials embedded in and covering the top bitumen coating. BUR is used primarily on commercial buildings which have flat or low-slope roofing systems. Its popularity arises from its relatively low cost combined with its effectiveness as a water repellant membrane and its durability.
Bituminous materials, especially asphalt and coal tars, have been utilized as roofing materials particularly in built-up roofing applications, as road paving materials, and as adhesive materials. However, such materials tend to deteriorate under conditions of extreme temperature variations, constant exposure to physical stress, and environmental weathering. Various proposals have been made in the art to modify the bituminous materials to overcome these deficiencies.
Although asphalt and coal tar have a similar appearance when in a package or in a finished roof, they are derived from different raw materials, and they are also different in their chemistry. Coal tar and asphalt generally are incompatible. Coal tar is obtained by heating bituminous coal to very high temperatures and collecting the volatile materials that are produced. These volatiles are referred to as crude coke oven tar, and the solid residue left behind is called coke. The crude coke oven tar is processed to recover a variety of materials including creosote and precursors for a large number of other important chemicals. The residue left after this processing step is called coal tar pitch which comprises primarily aromatic hydrocarbons. The coal tar pitch is the material used in the more familiar applications of roofing and asphalt concrete surface treating.
Asphalt, on the other hand, is derived from petroleum or crude oil and comprises primarily aliphatic hydrocarbons. Crude oil is processed at a refinery by distilling off the “light ends” to produce materials such as propane, gasoline, fuel oils, and chemical intermediates. The residue that remains from the distillation is referred to as straight-run asphalt. Straight run asphalt is processed primarily for road paving applications, and after further processing (e.g., oxidation or blowing), it is converted to roofing asphalts designated as D312 Types I through IV, by the American Society for Testing and Materials (ASTM).
Bituminous coatings are advantageously handled as aqueous emulsions since health and safety issues may be involved when volatile and combustible organic solvents are used in bituminous coatings. Water based coal tar emulsions have been described in the prior art as useful in preparing protective coatings. In its most common form, an aqueous coal tar emulsion comprises an emulsion of a coal tar in an aqueous medium with an emulsifying or dispersing agent such as an organic soap or detergent and/or an inorganic colloid such as a particulate clay. A conventional practice in the protective coating art is to brush, spray or paint the articles with an aqueous emulsion of coal tar. When the coating of coal tar dries on the article, it provides a protective film. Such films, however, have certain disadvantages such as being attacked by various solvents, being soft, and having inadequate elongation, ductility or tensile strength. It has been proposed to improve these properties by including various additives into the emulsions.